JP2018500592A - Light illuminator with optical path compensation for wavelength selective switch - Google Patents

Abstract

The optical device includes an optical port array, a first walk-off crystal, a first half-wave plate, a second walk-off crystal, and a segmented half-wave plate. The optical port array has a plurality of first and second ports for receiving a light beam. The first walk-off crystal spatially splits the beam into first and second portions that are in orthogonal first and second polarization states, respectively. The first part is walked off by the first walk-off crystal, and the second part passes without a walk-off. The first half-wave plate alternates the polarization state of the first and second portions of the light beam. The second walk-off crystal is oriented in the opposite direction to the first walk-off crystal, the second part is walked off by the second walk-off crystal, and the first part is walk-off. Pass without. A segmented half-wave plate receives the first or second portion of the light beam.

Description

  Optical networks use wavelength selective switches (WSS) for the dynamic routing of visible light signals from sources to targets. WSS devices often rely on wavelength-manipulating elements such as reflective liquid crystal panel (LCoS) devices or microelectromechanical system (MEMS) mirror arrays to perform routing.

  The LCoS device includes a liquid crystal material that is sandwiched between a transparent glass layer having a transparent electrode and a silicon substrate divided into a two-dimensional array of individually addressable pixels. Each pixel can be driven individually by a voltage signal and provides a local phase change to the optical signal, thereby providing a two-dimensional array of phase manipulation regions. The operation of individual spectral components is possible when the optical signal is spatially divided by a diffraction element such as a diffraction grating. The spatial division of the spectral components is directed to a predetermined region of the LCoS device and can be manipulated individually by driving the corresponding pixels in a predetermined manner.

  A method and apparatus are provided for compensating for differences in optical path lengths passed by first and second optical portions of a light beam in orthogonal polarization states. According to the method, a light beam is received at an input port and a first optical portion and a second optical portion in first and second polarization states, respectively, to spatially split the light beam into first and second optical portions. Directed to one walk-off crystal. The first polarization state and the second polarization state are orthogonal to each other. The first optical part is walked off by the walk-off crystal, and the second optical part passes without a walk-off. The polarization states of the first and second optical parts alternate. After the polarization states of the first and second optical portions are alternated, the first optical portion and the second optical portion are directed to the second walk-off crystal. Since the second walk-off crystal is oriented in the opposite direction to the first walk-off crystal, the second optical component is walked off by the walk-off crystal, and the first optical component is walk-off. Pass without. The thickness of the first walk-off crystal and the second walk-off crystal is selected to adjust the optical path length passed by each of the first optical portion and the second optical portion.

  In one specific embodiment, the optical device includes an optical port array, a first walk-off crystal, a first half-wave plate, a second walk-off crystal, and a segmented half-wave plate. Yes. The optical port array has a plurality of first ports for receiving a light beam and a plurality of second ports for receiving a light beam. The first walk-off crystal spatially splits each of the light beams into a first optical portion and a second optical portion that are in first and second polarization states, respectively. The first and second polarization states are orthogonal to each other. The first optical part is walked off by the first walk-off crystal, and the second optical part passes without a walk-off. The first half-wave plate alternates the polarization state of the first and second optical portions of the light beam. Since the second walk-off crystal is oriented in the opposite direction to the first walk-off crystal, the second optical portion is walked off by the second walk-off crystal, and the first optical portion is Pass without walk-off. The segmented half-wave plate receives the second optical portion of the light beam or the first optical portion of the light beam.

FIG. 6 shows a single input port to an optical arrangement such as WSS using LCoS devices. It is a figure which shows an example of an optical path compensation apparatus. FIG. 3 is a top view of an example of a simplified optical arrangement such as a free space in which the optical path compensation device shown in FIG. 2 can be employed. FIG. 3 is a side view of an example of a simplified optical arrangement such as a free space in which the optical path compensation device shown in FIG. 2 can be adopted.

  FIG. 1 shows a single input port 110 to an optical arrangement such as WSS using LCoS devices. The input beam 115 to such a device can often be very astigmatism. In the depiction of FIG. 1, the beam 115 has a small waist along the y-axis and a large waist along the z-axis. Randomly polarized beam 115 first enters walk-off crystal 120, which is split into two orthogonally polarized beams, walk-off beam 150 and pass beam 160. In FIG. 1, one polarization component (for example, vertical or v component) is indicated by a vertical arrow, and the other polarization component (for example, horizontal or h component) is indicated by a horizontal arrow. The walk-off direction of the walk-off crystal 120 and the direction of alternation caused by the half-wave plate 130 will be described with respect to the polarization component of the light beam propagating in the forward or downstream direction, ie, the positive z-direction.

  In order to bring both two spatially split beams 150 and 160 exiting the walk-off crystal 120 to the same polarization state, the walk-off beam 150 passes through the half-wave plate 130 and the half-wave plate 130. Alternates the walk-off beam from the h-polarization state to the v-polarization state. The v-polarized beam 160 does not pass through the half-wave plate 130. As a result, beams 150 and 160 are both in the same polarization state.

  One problem with the arrangement shown in FIG. 1 is that the two beams 150 and 160 propagate over different optical path lengths due to the walk-off crystal 120 and the half-wave plate 130. This can be a problem when the incident astigmatism beam has a small waist. For example, if the beam waist is about 3.5 microns, its Rayleigh length or range is about 30 microns. The path length difference experienced by the two beams is generally desirable to be less than this distance.

  In the example of FIG. 1, the walk-off beam 150 can travel an effective propagation distance of about 200 microns compared to the passing beam 160. Similarly, when passing through half-wave plate 130, walk-off beam 150 can travel an effective propagation distance of about 30 microns compared to pass beam 160. Thus, the difference in the total effective propagation distance traveled by the two beams is about 170 microns. Obviously, this distance is large compared to the beam Rayleigh range.

  One way to deal with this problem is to compensate for this difference in the total effective propagation distance by using two walk-off crystals oriented opposite to each other. In this case, a beam in one polarization state accumulates a difference in effective propagation distance in the first walk-off crystal, not in the second walk-off crystal, but a beam in the other polarization state The difference in effective propagation distance is accumulated in the second walk-off crystal, not in the first walk-off crystal. FIG. 2 shows an example of an optical path compensator operable by this method.

  As shown in FIG. 2, the walk-off beam 235 experiences an effective propagation distance difference of 90 microns with respect to the passing beam 225 in the first walk-off crystal 220. The half-wave plate 250 alternates the polarization state of the beam 225 from horizontal to vertical, so that the beam 225 will be walked off by the second walk-off crystal 240. Similarly, half-wave plate 250 alternates the polarization state of beam 235 from vertical to horizontal. As a result, beam 225 experiences an effective propagation distance difference of 120 microns with respect to beam 235 in the second walk-off crystal 240. In addition, beam 235 experiences an additional effective propagation distance difference of 30 microns relative to beam 225. This is because the beam 225 passes through the half-wave plate 230. Thus, beam 225 experiences a total effective propagation distance difference of 120 microns, and beam 235 also experiences a total effective propagation distance difference of 120 microns (ie, 90 + 30 microns). As a result, both beams propagate over the same path length.

  As shown in the example of the optical path compensator shown in FIG. 2, the thickness of the second walk-off crystal 240 is that of the first walk-off crystal 220 and the half-wave plate 230 through which only one beam passes. It is chosen to compensate for the difference between both effective propagation distances. Accordingly, the thickness of the two walk-off crystals 220 and 240 are different from each other, and this thickness determines the additional path length along which the walk-off beam passing through the walk-off crystal travels.

  FIG. 2 shows a single optical input port in an optical arrangement. A light beam received by a device using multiple ports is in a similar manner, in two orthogonal polarization states, and the spatially split beams that split each incident beam span the same path length. It can be processed to ensure propagation. If these optical arrangements include the functionality of a multi-wavelength switch, they can share a common set of optical elements such as lenses, dispersions, and spatial light modulators. An example of a wavelength selective switch using such methods and techniques will be described below in connection with FIGS. 3A and 3B.

  3A and 3B are top and side views, respectively, of an example of a simplified optical arrangement, such as a free space WSS 100 that can be used in connection with aspects of the present invention. Light is input to and output from the WSS 100 through an optical waveguide such as an optical fiber used as an input port and an output port. The fiber collimator array 101 includes a first fiber series 120 associated with a first WSS and a second fiber series 130 associated with a second WSS. Each individual fiber is associated with a collimator 102 that converts light from each fiber into a free space beam.

As best shown in FIG. 3B, the fibers 320 ₁ , 320 ₂ , 320 ₃ , 320 ₄ of the first fiber series 320 are the fibers 330 ₁ , 330 ₂ , 330 _{3 of} the second fiber series 330. And are arranged alternately. Further, as can be seen from FIG. 3B, the fibers of the fiber series 320 are offset with an angle from the fibers of the second fiber series 330. As shown in FIG. 2, an offset with this angle causes the wavelengths associated with two different WSSs to be spatially offset from each other in the y direction (port axis) on the LCoS device 21.

An optical path compensator of the type shown in FIG. 2 receives a light beam from each fiber / collimator pair. 2 and 3, similar elements are given the same reference numerals. FIG 3B, are shown two typical beam, the first beam is received by the fiber 320 ₁ (associated with the first WSS), the second beam (second associated with WSS) is received by adjacent fibers 330 _1. As shown, after exiting the first walk-off crystal 220, the light beam received by the fiber 320 _1, is divided into two beams 323 and 325, these beams are orthogonally polarized with respect to each other Is in a state. Similarly, after exiting the second walk-off crystal 240, the light beam received by the fiber 330 _1, is divided into two beams 333 and 335, these beams are in orthogonal polarization states with respect to one another.

The beams from the two different WSSs are angled so that they intersect in the plane of the patterned half wave plate 260 with the half wave plate segment 230. Thus, the position of the patterned half-wave plate determines the angle at which the beam must be emitted from the optical port. In the example of FIG. 3B, the beam 323 from port 320 ₁ associated with the first WSS and the beam 333 from port 130 ₁ associated with the second WSS intersect at half-wave plate segment 230. doing. Similarly, beam 325 from port 320 ₁ associated with the first WSS and beam 335 from port 330 ₁ associated with the second WSS intersect at half-wave plate segment 230. . In one alternative embodiment, beams 325 and 335 can be directed to half-wave plate 230 rather than beams 323 and 333.

  The following optical path compensator, telescope pair, or light beam expander expands the free space light beam from the port array 101. The first telescope or beam expander is formed from optical elements 106 and 107, and the second telescope or beam expander is formed from optical elements 104 and 105.

  In FIGS. 3A and 3B, an optical element that acts on light in two axes is shown in solid lines as a biconvex optical element in both drawings. On the other hand, an optical element that acts on light only in one axis is shown as a plano-convex lens by a solid line on the axis that acts. An optical element that acts on light only in one axis is also indicated by a broken line on an axis where the optical element does not act. For example, in both FIGS. 3A and 3B, the optical elements 102, 108, 109, 110 are shown in solid lines. On the other hand, the optical elements 106 and 107 are indicated by solid lines in FIG. 3A (since the optical elements have the ability to focus along the y-axis), and are indicated by broken lines in FIG. (Because the beam is not affected along the x axis). The optical elements 104, 105 are shown in solid lines in FIG. 3B (since the optical elements have the ability to focus along the x axis), and are shown in broken lines in FIG. 3A (the optical elements are (Because the beam is not affected along the y axis).

  Each telescope may be formed with a different magnification factor in the x and y directions. For example, the magnification of a telescope formed from optical elements 104 and 105 that expand light in the x direction may be smaller than the magnification of a telescope formed from optical elements 106 and 107 that expand light in the y direction.

  The pair of telescopes expands the light beam from the port array 101 and optically couples the light beam to a wavelength dispersive element 108 (eg, a diffraction grating or a prism). The wavelength dispersive element 108 Divide into its constituent wavelengths or channels. The wavelength dispersion element 108 acts to disperse light in different directions in the xy plane according to the wavelength. Light from the dispersive element is directed to the beam focusing optical element 109.

The beam focusing optical element 109 connects the wavelength component from the wavelength dispersion element 108 to an optical phase modulator that can be controlled by a program. The optical phase modulator may be a liquid crystal based phase modulator, such as an LCoS device 110, for example. A program-controllable optical phase modulator produces a phase shift in each of its pixels, which phase shift across its surface produces a phase shift profile. As shown in FIG. 3, the wavelength components are dispersed along the x-axis. Thus, each wavelength component of a given wavelength is focused on an array of pixels 19 extending in the y direction. As an example, FIG. 3A shows three wavelength components focused on the LCoS device 110 along the chromatic dispersion axis (x-axis) with the center wavelengths indicated by λ _1, λ ₂ , and λ _3. Is not intended to be limiting.

  As best seen in FIG. 3B, after reflection from the LCoS device 110, each wavelength component passes through the beam focusing optical element 109, the wavelength dispersive element 108, and the optical elements 106, 107 in the port array 101. It can be coupled back to the selected fiber. Thus, proper manipulation of the pixel 19 in the y-axis allows each wavelength component to be selectively and individually guided to the selected output fiber.

  Although the subject matter has been described in terms specific to structural features and / or methodological acts, it is understood that the subject matter recited in the claims is not necessarily limited to the specific features or acts described above. Should.

19 pixels 100 free space WSS
DESCRIPTION OF SYMBOLS 101 Fiber collimator array, port array 102 Collimator 102,104,105,106,107,108,109,110 Optical element 108 Wavelength dispersion element 109 Beam focusing optical element 110 Input port, optical element, LCoS device 115 Input beam, Beam 120 Walk-off crystal, first fiber series 130 half-wave plate, second fiber series 150 walk-off beam, beam 160 passing beam, polarized beam, beam 220 first walk-off crystal 225 passing beam, beam 230 half-wave plate, half-wave plate segments 235 walk-off beam, the beam 240 a second walk-off crystal 250 half-wave plate 260 half-wave plate 320 first fiber series 320 _{1, 320 2,} 320 3, 320 ₄ fiber 320 Port 323,325,333,335 beam 330 second fiber series 330 _{1, 330} 2, 330 ₃ fiber 330 ₁ port lambda _1, lambda _2, lambda ₃ center wavelength

Claims (18)

A method for compensating for a difference in optical path lengths passed by a first optical portion and a second optical portion of a light beam in an orthogonal polarization state, comprising:
Receiving the light beam at an input port;
The light is applied to a first walk-off crystal to spatially split the light beam into the first optical portion and the second optical portion that are in a first polarization state and a second polarization state, respectively. Directing a beam, wherein the first polarization state and the second polarization state are orthogonal to each other, and the first optical portion is walked off by a walk-off crystal, Passing the optical part without walk-off;
Alternating polarization states of the first optical portion and the second optical portion; and after the polarization states of the first optical portion and the second optical portion are alternated, the first optical portion and Directing the second optical portion to a second walk-off crystal, since the second walk-off crystal is oriented in a direction opposite to the first walk-off crystal, A second optical component is walked off by the walk-off crystal, the first optical component passes without a walk-off, and the thickness of the first walk-off crystal and the second walk-off crystal is , Selected to adjust the optical path length passed by each of the first optical portion and the second optical portion.   Furthermore, the thickness of the first walk-off crystal and the second walk-off crystal arises from one or more upstream or downstream optical elements, and the first optical portion and the second optical portion The method of claim 1, wherein the method is selected to compensate for optical path differences that are passed by one of the two and not passed by the other of the first optical portion and the second optical portion.   The method according to claim 1, wherein the optical element is a half-wave plate disposed downstream of the first walk-off crystal and the second walk-off crystal.   The method of claim 1, wherein the first input port is offset with an angle about an axis parallel to the chromatic dispersion axis with respect to the second input port. A method of directing a wavelength component of a light beam from an input port of a port array to at least one output port of the port array,
Receiving a first light beam at a first input port of the port array associated with a first wavelength selective switch;
Receiving a second light beam at a second input port of the port array associated with a second wavelength selective switch;
In order to spatially split the first light beam into a first optical part and a second optical part in a first polarization state and a second polarization state, respectively, Directing a first light beam, wherein the first polarization state and the second polarization state are orthogonal to each other, and the first optical portion is walked off by a walk-off crystal; Passing the second optical part without a walk-off;
In the first walk-off crystal, the second light beam is spatially divided into a third optical portion and a fourth optical portion in a first polarization state and a second polarization state, respectively. Directing the second light beam, wherein the third optical portion is walked off by the walk-off crystal, and the fourth optical portion passes without a walk-off;
Alternating polarization states of the first optical portion, the second optical portion, the third optical portion, and the fourth optical portion;
After the polarization states of the first optical part, the second optical part, the third optical part, and the fourth optical part are changed, the first optical part and the second optical part Directing the third optical portion and the fourth optical portion to a second walk-off crystal, wherein the second walk-off crystal is opposite to the first walk-off crystal. The second optical portion and the fourth optical portion are walked off by the second walk-off crystal, and the first optical component and the third optical component are The walk-off crystal passes through each of the first optical portion, the second optical portion, the third optical portion, and the fourth optical portion. To adjust the optical path length Steps to be-option;
Spatially dividing the wavelength components of the first optical portion, the second optical portion, the third optical portion, and the fourth optical portion;
The focus of the wavelength component that is spatially divided is adjusted to an optical phase modulator that can be controlled by a program, and the wavelength components of the first light beam and the second light beam are aligned with the wavelength dispersion axis of the modulator. Spatially dividing along: and
Adjusting the phase shift profile of the modulator along a second direction to selectively direct one of the wavelength components to an output port.   The thickness of the first walk-off crystal and the second walk-off crystal further arises from one or more upstream or downstream optical elements, wherein the first optical portion and the second optical portion Is passed by one of the first optical portion and the second optical portion is not passed by the other, but is passed by one of the third optical portion and the fourth optical portion. 6. The method of claim 5, wherein the method is selected to compensate for a difference in optical path that is not passed by the other of the third optical portion and the fourth optical portion.   The method according to claim 5, wherein the optical element is a half-wave plate disposed downstream of the first walk-off crystal and the second walk-off crystal.   The method of claim 5, wherein the first input port is offset with respect to the second input port at an angle about an axis parallel to the chromatic dispersion axis. An optical port array having a plurality of first ports for receiving light beams and a plurality of second ports for receiving light beams;
A first walk-off crystal that spatially splits each of the light beams into a first optical portion and a second optical portion in a first polarization state and a second polarization state, wherein And the second polarization state are orthogonal to each other, the first optical portion is walked off by the first walk-off crystal, and the second optical portion is not walked off. A first walk-off crystal passing through;
A first half-wave plate that alternates the polarization state of the first optical portion and the second optical portion of the light beam;
A second walk-off crystal oriented in a direction opposite to the first walk-off crystal, wherein the second optical portion is walked off by the second walk-off crystal, A second walk-off crystal through which the optical portion passes without being walked off; and
An optical device comprising a segmented half-wave plate for receiving the second optical portion of the light beam or the first optical portion of the light beam.   The thickness of the first walk-off crystal and the second walk-off crystal adjusts the optical path length passed by the first optical portion and the second optical portion, and the segmented half-wavelength The first optical part or the second optical part passing through a plate and the other of the first optical part and the second optical part not passing through the segmented half-wave plate; The optical device of claim 9, wherein the optical device is selected to compensate for the resulting optical path difference. Further, the first optical portion and the second optical portion of the light beam are received, and the first optical portion and the second optical portion are spatially separated into a plurality of wavelength components along a wavelength dispersion axis. And the port axis is orthogonal to the chromatic dispersion axis, and the plurality of first ports extend in a first plane including the port axis, and The plurality of second ports extend in a second plane including the port axis, and the first plane and the second plane are parallel to each other and along the wavelength dispersion axis The plurality of first ports are offset with respect to the plurality of second input ports at an angle around an axis parallel to the wavelength dispersion axis. Dispersive element;
A focusing element for focusing a plurality of wavelength components; and
A program-controllable optical phase modulator for receiving a plurality of focused wavelength components, the modulator receiving a wavelength component received from any one of the plurality of first ports , And configured to direct to a selected one of the plurality of first ports, and further, the wavelength component received from any one of the plurality of second ports, The optical device of claim 9, comprising an optical phase modulator configured to direct to a selected one of the plurality of second ports.   The optical device according to claim 9, wherein the plurality of first ports are alternately arranged with the plurality of second ports along the port axis.   The optical device of claim 9, wherein the optical phase modulator controllable by a program comprises a liquid crystal based phase modulator.   The optical device of claim 13, wherein the liquid crystal based phase modulator is an LCoS device.   The optical device of claim 11, wherein the dispersive element is selected from the group consisting of a diffraction grating and a prism.   The optical device according to claim 11, further comprising an optical system for expanding the light beam received from the optical port array and directing the expanded light beam toward the dispersive element.   The optical system has a first magnification factor in a first direction, a second magnification factor in a second direction orthogonal to the first direction, and the first magnification factor is The optical device of claim 16, wherein the optical device is different from the second magnification factor.   The first direction is parallel to a chromatic dispersion axis, the light beam is spatially divided along the chromatic dispersion axis, and the first expansion coefficient is greater than the second expansion coefficient. The optical device according to claim 17, which is also small.

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